Secondary electron emitter



1 58 w. G. SHEPHERD 2,846,338 SECONDARY ELECTRON EMITTER Filed Aug. 5. 1954 s Sheets-Sheet 1 JNVENTOR.

WILLIAM G. SHEPHERD 3 Sheets-Sheet 2 Filed Aug. '3. 1954 FIG. 3

FIG. 5

INVENTOR. WILLIAM G. SHEPHERD ATTORNEY sncoNnARY ELECTRON EMITTER William G. Shepherd, St. Paul, Minru, assignor to the United States of America as represented by the Secretary of the Army Application August 3, 1954, Serial No. 447,691

3 Claims. (Cl. 117-219) This invention relates to electron emissive surfaces and more particularly to a secondary emitting surface having .a substantially unvarying ratio of secondary current to primary current for a relatively long life period.

It is well known that secondary emitting magnesium oxide films are eroded away very quickly at the point of primary electron bombardment. This factor decreases the amount of secondary electrons emitted per absorbed primary and accordingly the effective useful life of the surface is very short.

It is accordingly the primary object of the present invention to provide a means for protecting a magnesium oxide secondary against the erosion caused by electron bombardment and providing a surface having essentially the same secondary emission as the original surface but having a substantially unvarying ratio of secondary current to primary current for a relatively long life period.

In accordance with the present invention, there is provided a secondary emitting coating for an electrode characterized by a substantially unvarying ratio of secondary emission to absorbed primary for a relatively long life comprising an underlying film of a material which is unstable when subjected to electron bombardment and a thin film of material which is stable when subjected to electron bombardment over said unstable material, said stable material preventing the bombardment produced dissociation products in said unstable material from escaping, thereby permitting their recombination and a consequent increased life of the underlying film.

Also, in accordance with the present invention there is provided an electrode characterized by a substantially unvarying ratio of secondary emission to absorbed primary for a relatively long life comprising a body consisting of magnesium-silver alloy, a thin film of magnesium oxide on said body, a layer of barium oxide on said magnesium oxide, said barium oxide being of such thickness that the secondary emission ratio of the coating is substantially equal to the secondary emission ratio of said magnesium oxide film. 4

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawing Fig. 1 is a front elevation, partly in section of a tube structure for providing the coating of the present invention,

Fig. 2 is a side elevation of the tube of Fig. 1,

Fig. 3 illustrates components of a tube utilized in testing the coating provided in the tube of Fig. 1,

Fig. 4 is a group of curves illustrating the influence of a barium oxide overlayer on the secondary emission characteristics of a magnesium oxide film.

Fig. 5 is a group of curves illustrating a comparison of the efficacy of the present invention compared to prior art coatings.

Referring now more particularly to Fig. 1, there is States Patent M 2,846,333 Patented Aug. 5, 1958 shown a glass container 2 comprising two separate chambers 4 and 6 and side arms 8 and 10. Contained in chamber 4 is an electron gun assembly 12 for providing primary electrons, the applied heater potentials for assembly 12 not being shown. Spaced from assembly 12 is an electron collector 14 which is a substantially hemispherical dish having an opening 16 in its periphery perpendicular to the electron path. Collector 14 is biased several volts positive with respect to assembly 12 (biasing potentials not shown), so as to draw the electrons therethrough where they impinge on a target electrode 18 mounted on a movable trolley 20. Movable trolley 20 consists in this embodiment of tungsten wires sliding in short nickel tubes and fitted with iron slugs at each end. With this arrangement, by using an external magnet (not shown), target electrode may be moved into side arm 10 for secondary emitting coating formation. Movable trolley 20 permits the processing and testing of several target electrodes in sequence. Chamber 6 terminates in side arm 10 and has contained therein barium carbonate coated cathode 22, spaced from trolley 20, the evaporation product of cathode 22 forming on a target electrode 18 when cathode 22' is heated by R. F. heating or other suitable heating means (not shown).

In Fig. 2, there can be seen auxiliary side arms 24 and 26 extending from the junction of chamber 6 and side arm 10. In auxiliary side arm 26 there is contained an assembly 28 of several nickel strips, eight in the present embodiment, to, check the rate of barium evaporation from cathode 22 during testing.

To process target electrode 18, the following procedure was pursued. The targets were made of 1.7% magnesiumsilver alloy, with dimensions .375 x .375" x .005". Target 18 was cleaned mechanically using fine aluminum oxide as a polishing agent and then degreased with acetone. After cleaning, target'18 was mounted in tube 2 and the tube was pumped with a mercury diffusion pump. The system was baked out at 450 C. until the pressure fell to less than 5 10- mm. of mercury. The system was permitted to cool whereupon the pressure fell to less than 1X10 mm. of mercury. Electrons were drawn from-assembly 12, the secondary emission ratio was measured, and a mercury valve (not shown) was closed to seal the tube. Pure oxygen was admitted into the tube, and target electrode 18 was oxidized by heating with radio frequency induction.

Target electrode 18 was oxidized at temperatures of about 750 C. (brightness temperature) in pure oxygen at a pressure of 5 to 6 microns for a period of about 2 minutes. After oxidation, target electrode 18 was outgassed for several minutes at a temperature in excess of 600 C. The secondary emission ratio was then investigated to determine 6 6 being the ratio of produced secondary electrons per absorbed primary.

After the preparation of a target electrode 18 having a magnesium oxide secondary emitting surface thereon, it was transferred to tube 30 shown in Fig. 3.

Tube 30 was evacuated on a mercury diffusion pump station and the bake-out temperature was raised slowly to 400 C. so that the pressure in the system did not exceed 2X10- mm. of mercury thus preventing contamination of the secondary emitting surface. When the pressure had fallen to less than 1 10 mm. of mercury, the bake-out oven was shut off. The cathode 32 of tube 30 was then activated, the tube was sealed off and the getter (not shown) was flashed.

Tube 30 was constructed to provide a relatively high density beam of electrons emitted from an oxide coated cathode 32 in an electron gun assembly 34. In order to clearly distinguish between efiects due to electron bombardment and those caused by evaporation from cathode '32, the target electrode 18 was located so that it could not see the cathode. Electron gun assembly 34 produced an approximately cylindrical beam which passed through a grid 35 and which was defiectedby electrodes 36, 38 and .40 to' strikedhe 'surface rot target electrode 18. 'The' striking point'ot' the electron beam could be shifted from one portion Of-Jthe target surface by adjustment of the'potentials. This"m'ade' possihle'a reference surfacev which .was not under continuous home bardment so that effects due to the bombardment. could be distinguished from general changes of the target I Several tubes such as tube 30 containing atargetlsucli as target electrode 18 were life tested by bombarding the surface of target 18 with a current density of about ma/cm. for periods ranging in excess of about 750 hours, After the completion of the life test, the tubes were opened and surfaces onthe several target electrodes 18 were given a visual inspection. In each case it was found that the magnesium oxide films had been eroded away at 'the'point of electron bombardment. It hasbeen found as a result of the present invention that a barium oxide coating of 4to 8 molecular layers deposited upon thin magnesium oxide secondary emitting films has no effect upon the quantity of secondary emission from the magnesium oxide film as compared to a clean magnesium oxide surface but yet serves as a fence to prevent the dissociation products of magnesium oxide produced during electron bombardment from escaping.

To provide .the'surface of the invention, magnesium thin film samples were prepared with an overlayer of 4- -8 molecularilayersof barium oxide by preparing magnesium oxide coated target electrodes 18 in accordance with the 4 molecular layers of barium oxide and decreases monatornically'forthicker" overlayers.

In Fig. 5, there are shown curves illustrating the protective efiect of such a layer of barium oxide coating, over magnesium oxide in enhancing the life of the magnesium oxide. The abscissa is the time of bombardment of the coating in hours and the ordinates are o wherein '6 is the initial secondary to primary emission ratio and 6 is the secondary to primary emission ratio during the tests Curve A represents 'the results for a magnesium oxide surface, bombarded at a current density of 10 ma./cm. ,=and the initial value 6 is 3.8. Curves B and C show typical surfaces bombarded at 16 ma./cm. and 50 ma./cm. with 6 equal to 3.6 and 4.0 respectively. Curvew D illustratesithehresults.for the magnesium oxide coating havingifourmolecular-layers of barium oxide thereon tbombarded'with a current 60 Ina/cm} and 6 :11. it is to be noted that :the time required for this surfaceto fallto 80%--of itsiinitial value. is about ,190

hours-:Itis to be seen that although the current density procedure outlined in the treatment in tube 2 and then exposing the prepared electrodes'to a barium oxide cathode of chamber 6 in tube 2. The amount of layers of barium ioxide provided on the magnesium oxide layer may. be calculated. from the Weight of barium oxide deposited and the lattice structure of barium oxide. .The t determination of the weights of such small amounts of barium'oxi'dernay be made by using a radioactive barium isotope and examination of radiation from thelta'rget electrode. with a Geiger counter. The use of radioactive elements in such determination is discussed in Use 'of Radioactive Elements, in the Study of OxidefCathode's byJean -Debie'sse'in LOnde Electrique, vol. 30, August 1950'; pages :35 1-353.

The weight of the coatings may be determined from the densityof barium oxide and it's lattice spacing. 'Fro'rh these;one may calculate thata monolayer of barium .oxide has a mass of 1.58 XlO-" grams per square centimeter. Themes of the layer is proportional 'to its thickness.

Fig.4 shows the influence of a barium oxide overlayer'on the secondary emission characteristics of a'm'agnesium oxide film. It will be seen that the secondary emission of the compositesurface is enhanced over the simple magnesium oxide surface when the overlayer of barium oxide is approximately 2 molecular layersflfllick,

has the same value .as the simple surface at approximately of curve; D is sixitimesthat of curve A, curve'A falls to-'8 0% ofits initial value; in little more-than hours.

awhileqtherehas. been described; what is, at present, considered to be the preferred embodimentof the invention,,-it will be obvious to those skilled-in the art that yariousachanges; and modificationsmayv be made therein without departing fromlthe. invention and it is, therefore, aimed in theappended claims to cover all suchchanges andmodifications-as fallwithinthe true spirit and scope ofrthe invention. What is claimed is: V

-.1.-A secondary welectron-emitter comprising a body consistingtof a magnesium silven'all0y,a duplex coating comprising athin fihnof magnesium oxide on said body and.alayer. of barium 'oxideon saidv magnesium oxide, -said bar-ium--oxide bein of suchthicknessthat the secondaryemissionratio of the coating is substantially equal to thezsecondaryemissionmatio of saidmagnesium oxide layers.

References Cited in the file of this patent UNITED STATES PATENTS 'Wilson; June 13,1922 2,147,669 Piore .L. Feb. 21, 1939 2,198,329 Bruining etal. Apr. 23, 1940 2,205,055 Zvvorykin June 18, .1940 2,620,287 Bramley Dec.v 2', 1952 2,657,325 Homer et al. J. Oct. 27, 1953 

1. A SECONDARY ELECTRON EMITTER COMPRISING A BODY CONSISTING OF A MAGNESIUM-SILVER ALLOY, A DUPLEX COATING COMPRISING A THIN FILM OF MAGNESIUM OXIDE ON SAID BODY AND A LAYER OF BARIUM OXIDE ON SAID MAGNESIUM OXIDE, SAID BARIUM OXIDE BEING OF SUCH THICKNESS THAT THE SECONDARY EMISSION RATIO OF THE COATING IS SUBSTANTIALLY EQUAL TO THE SECONDARY EMISSION RATIO OF SAID MAGNESIUM OXIDE FILM. 